1. Field of the Invention
The invention relates to the field of apparatuses for orientation of solar radiation collectors.
2. Related Art
Existing active solar trackers are usually based on electrooptical solar sensors as described for instance in U.S. Pat. Nos. 3,493,765; 4,223,214; 4,328,417; and 5,317,145. Such trackers can work with a high accuracy. These trackers are complex and, therefore, expensive and unreliable. Electrooptical solar trackers are usually composed of at least one pair of antiparallel connected photoresistors or photovoltaic solar cells which are electrically balanced by an equal intensity of illumination of both elements so that there is either no or negligible control signal on a driving motor. A differential control signal, occurs due to differential illumination of the electrooptical sensor, and is used to drive a motor and to orient an apparatus in such direction where illumination of the electrooptical sensors is equal and a balance is restored.
There are other active solar trackers based on clockworks and/or combining both principles as described in U.S. Pat. No. 4,031,385. Such trackers can work with high accuracy but they are complex and, therefore, expensive and unreliable.
Existing passive solar trackers are based on thermal expansion of matter and/or on shape memory alloys. They are usually composed of a pair of actuators working against each other which are balance by equal illumination. An unbalance of forces caused by a differential illumination of the actuators is used for orientation of the apparatus in a direction such that there is equal illumination of actuators and a balance of forces is restored, as described e.g., in U.S. Pat. Nos. 2,967,249; 4,027,651; GB Patent 1,566,797; CZ Patent 279,801 and DE 33 03 000 A1. Passive solar trackers compared to active trackers are less complex and less expensive but they work with low efficiency and they do not work at all at low temperatures.
Other active solar trackers are described in EP-A-0 050 189 and WO-A-84/00068. The apparatus described in EP-A-0 050 189 has detectors, such as photosensors with inclined sunbeam receiving surfaces, issuing a detecting signal to drive a motor so that the apparatus is always directed to the sun. The detectors are not parallel to each other. The apparatus described in WO-A-84100068 has two solar cells that are arranged parallel side by side but with their active (sunbeam receiving) surfaces oriented in the same direction.
Both active and passive solar trackers use a differential illumination of sensors and actuators, shadowing means, mirrors or lenses, and/or an arrangement of sensors or actuators to each other, and/or a combination of the above mentioned means as it is described e.g., in patents U.S. Pat. Nos. 4,082,947; 1,474,397; and DE 43 06 656 A1.
For instance, two axis solar tracking is reached by sensing solar cells placed symmetrically on concurrent faces of a pyramid or truncated pyramid while an axis of symmetry is defined by the vertex of the pyramid and by the sun. A single axis solar tracking is reached by sensing solar cells placed symmetrically on concurrent faces of a triangular prism or truncated triangular prism while a plane of symmetry is defined by the intersection of concurrent planes and by the sun.
Concurrent symmetrical arrangement of antiparallel connected solar cells is advantageous for accurate tracking of the sun as it compensates isotropic and circumsolar diffuse parts of solar radiation that decrease an accuracy of tracking. A direct radiation only is used for tracking of the sun in this case.
A disadvantage of this arrangement is reduction of power of solar cells as a diffuse circumsolar radiation represents a substantial part of the solar radiation energy.
A total output power of concurrently arranged antiparallel connected solar cells is further decreased as only a differential power can be used for tracking. A low efficiency of concurrently arranged antiparallel connected solar cells substantially increases a price of the apparatus as a price of solar cells represents a substantial part of price of the apparatus.
A further disadvantage of concurrently arranged antiparallel connected solar cells is late backtracking of solar collectors in the morning as the apparatus that finished tracking in the preceding day afternoon has no solar cell oriented in the direction contrary to the direction of sun movement. This is the reason why backtracking starts at late morning when the sun is high enough above horizon. Use of auxiliary solar cells for backtracking increases price and complexity of the apparatus. Use of curved solar cells increases the price too. Bigger cells have to be used for the same output power due to the fact that only a part of cells is illuminated. Besides, the partial illumination could damage solar cells.